[fpc-devel] [Suggestion] Enumeration range-check intrinsic

[J. Gareth Moreton]

It feels like a very convoluted way to implement a safe means of reading 
an enumeration from a file, but given that that reading incorrect data 
files is one of the main causes of enums taking on invalid values, I 
have to say that this is the best solution should we not be allowed the 
"is" operator or an intrinsic.  If that is the case, then I propose this 
gets added to TStream!

Gareth aka. Kit

On 05/07/2019 21:27, Sven Barth via fpc-devel wrote:
> Am 05.07.2019 um 22:25 schrieb Sven Barth:
>> Am 05.07.2019 um 08:08 schrieb Sven Barth:
>>> Jonas Maebe <jonas at freepascal.org <mailto:jonas at freepascal.org>> 
>>> schrieb am Do., 4. Juli 2019, 21:21:
>>>
>>>     On 03/07/2019 09:26, Ondrej Pokorny wrote:
>>>     > On 02.07.2019 23:34, Jonas Maebe wrote:
>>>     >> Invalid data means undefined behaviour, always. "is" is not a
>>>     special
>>>     >> case that is immune to this.
>>>     >
>>>     > Don't you really see the need to handle invalid data with a
>>>     /defined/
>>>     > behavior?
>>>
>>>     My point is that is impossible to do so, so trying to do it in a way
>>>     that works in some/most cases, is much more dangerous than
>>>     categorically
>>>     refusing to try to do it, as it creates a false sense of security.
>>>
>>>
>>> Then how would you read data from e.g. a stream into an enum or 
>>> subrange if the stream may contain invalid data?
>> I now did a proof of concept for that task myself:
>>
>> === code begin ===
>>
> [snip]
>> if not s.specialize ReadEnum<TMyEnum>(e) then
>>         raise EStreamError.Create('Failed to read enum value');
>>       Writeln('Read value: ', e);
>>       if not s.specialize ReadEnum<TMyEnum>(e) then
>>         raise EstreamError.CReate('Failed to read enum value');
> [snip]
>>
>> === code end ===
>
> Oh and once I've integrated Ryan's implicit specialization support the 
> code can become this (or at least I hope so ^^'):
>
> === code begin ===
>       if not s.ReadEnum(e) then
>         raise EStreamError.Create('Failed to read enum value');
>       Writeln('Read value: ', e);
>       if not s.ReadEnum(e) then
>         raise EstreamError.CReate('Failed to read enum value');
>
> === code end ===
>
> Regards,
> Sven
>
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